The 521 sold out in four days at the first retailer to carry it, the Home Shopping Network.

The 521 is a T-Mobile-specific variant of the Lumia 520 that debuted at Mobile World Congress in February. It offers good-but-average hardware for the price (1 GHz dual-core processor, 4-inch touch screen, a 5-megapixel camera, 8G storage expandable to 64G).

However, the phone doesn't require a long-term contract. Walmart will sell it with T-Mobile's $30/month unlimited data and text plan.

If this phone sells well at Walmart, Microsoft may have found a way to attract more users to its Windows Phone platform. The more users it has, the more app developers it will attract. More apps will bring in more users.